Fix typo [skip ci]

astroquery/xmatch/core.py

@@ -147,7 +147,7 @@ def _prepare_sending_table(self, cat_index, payload, kwargs, cat, colRA, colDec)
         if not self.is_table_available(cat):
             if ((colRA is None) or (colDec is None)):
                 raise ValueError(
-                    f"'{cat}' is not available on the XMatch server.If you are "
+                    f"'{cat}' is not available on the XMatch server. If you are "
                     "using a VizieR table name, note that only tables with "
                     "coordinates are available on the XMatch server. If you are "
                     f"using a local table, the arguments 'colRA{cat_index}' and "